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          <th colspan="3" align="center">Chapter 3. The DB Replication Framework</th>
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      <div class="titlepage">
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          <div>
            <h2 class="title"><a id="repapp"></a>Chapter 3. The DB Replication Framework</h2>
          </div>
        </div>
        <div></div>
      </div>
      <div class="toc">
        <p>
          <b>Table of Contents</b>
        </p>
        <dl>
          <dt>
            <span class="sect1">
              <a href="repapp.html#rep_init_code">
                Starting and Stopping Replication
            </a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="repapp.html#election_flags">Managing Election Policies</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="repapp.html#thread_count">Selecting the Number of Threads</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="repmgr_init_example_c.html">Adding the Replication Framework to
                    
                    SimpleTxn
            </a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="fwrkpermmessage.html">Permanent Message Handling</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="fwrkpermmessage.html#fmwrkpermpolicy">Identifying Permanent Message Policies</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="fwrkpermmessage.html#fmwrkpermtimeout">Setting the Permanent Message Timeout</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="fwrkpermmessage.html#perm2fmwrkexample">Adding a Permanent Message Policy to 
                            
                            
                            RepQuoteExample
                    </a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="electiontimes.html">Managing Election Times</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="electiontimes.html#electiontimeout">Managing Election Timeouts</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="electiontimes.html#electretrytime">Managing Election Retry Times</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="fmwrkconnectretry.html">Managing Connection Retries</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="heartbeats.html">Managing Heartbeats</a>
            </span>
          </dt>
        </dl>
      </div>
      <p>
        The easiest way to add replication to your transactional
        application is to use the replication framework. The replication framework provides a comprehensive 
        communications layer that enables replication. For a brief listing
        of the replication framework's feature set, see 
        <a href="apioverview.html#repframeworkoverview">Replication Framework Overview</a>.
    </p>
      <p>
            To use the replication framework, you make use of special methods off the
            <tt class="classname">Environment</tt> and
            <tt class="methodname">EnvironmentConfig</tt>
            classes. You also use a series of related classes to perform
            your implementation. For example, in order to detect whether
            your code is running as a master or a replica, you must
            implement <tt class="classname">com.sleepycat.db.EventHandler</tt>.
            (see <a href="fwrkmasterreplica.html#determinestate">Determining State</a>).
            That is:
    </p>
      <div class="orderedlist">
        <ol type="1">
          <li>
            <p>
                                    Create an environment handle as normal.
                            </p>
          </li>
          <li>
            <p>
                                    Configure your environment handle as
                                    needed (e.g. set the error file and
                                    error prefix values, if desired).
                            </p>
          </li>
          <li>
            <p>
                                    Use the replication framework replication methods to
                                    configure the replication framework. Using these
                                    methods causes DB to know that you
                                    are using the replication framework.
                            </p>
            <p>
                                    Configuring the replication framework 
                                    entails setting its replication
                                    priority, setting the TCP/IP address
                                    that this replication environment will use for
                                    incoming replication messages, identify
                                    TCP/IP addresses of other replication
                                    environments, setting the number of
                                    replication environments in the
                                    replication group, and so forth. These actions are
                                    discussed throughout the remainder of
                                    this chapter.
                            </p>
          </li>
          <li>
            <p>
                                    Open your environment handle. When you
                                    do this, be sure to specify 

                                    
                                    
                                    <span>
                                    <tt class="methodname">EnvironmentConfig.setInitializeReplication()</tt>
                                    when you configure your environment
                                    handle. This is in addition to the
                                    configuration that you would normally
                                    use for a transactional application.
                                    This causes replication to be
                                    initialized for the application.
                                    </span>
                            </p>
          </li>
          <li>
            <p>
                                    Start replication by calling
                                    
                                    
                                    <span><tt class="methodname">Environment.replicationManagerStart()</tt>.</span>
                            </p>
          </li>
          <li>
            <p>
                                    Open your databases as needed. Masters
                                    must open their databases for read
                                    and write activity. Replicas can open
                                    their databases for read-only activity, but
                                    doing so means they must re-open the
                                    databases if the replica ever becomes a
                                    master.  Either way, replicas should never attempt to
                                    write to the database(s) directly.
                            </p>
          </li>
        </ol>
      </div>
      <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
        <h3 class="title">Note</h3>
        <p>
                            The replication framework allows you to only use one
                            environment handle per process.
                    </p>
      </div>
      <p>
                    When you are ready to shut down your application: 
            </p>
      <div class="orderedlist">
        <ol type="1">
          <li>
            <p>
                                    Close your databases
                            </p>
          </li>
          <li>
            <p>
                                    Close your environment. This causes
                                    replication to stop as well.
                            </p>
          </li>
        </ol>
      </div>
      <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
        <h3 class="title">Note</h3>
        <p>
                            Before you can use the replication framework, you may have to
                            enable it in your DB library. This is
                            <span class="emphasis"><em>not</em></span> a requirement for
                            Microsoft Windows systems, or Unix systems that
                            use pthread mutexes by default. Other systems,
                            notably BSD and BSD-derived systems (such as
                            Mac OS X), must enable the replication framework when you
                            configure the DB build.
                    </p>
        <p>
                            You do this by <span class="emphasis"><em>not</em></span>
                            disabling replication and by configuring the
                            library with POSIX threads support. In other
                            words, replication must be turned on in the
                            build (it is by default), and POSIX thread
                            support must be enabled if it is not already by
                            default.  To do this, use the
                            <tt class="literal">--enable-pthread_api</tt> switch
                            on the configure script.
                    </p>
        <p>
                        For example:
                    </p>
        <pre class="programlisting">../dist/configure --enable-pthread-api</pre>
      </div>
      <div class="sect1" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h2 class="title" style="clear: both"><a id="rep_init_code"></a>
                Starting and Stopping Replication
            </h2>
            </div>
          </div>
          <div></div>
        </div>
        <p>
                    As described above, you introduce replication to an
                    application by starting with a transactional
                    application, performing some basic replication
                    configuration, and then starting replication using 
                    
                    
                    <span><tt class="methodname">Environment.replicationManagerStart()</tt>.</span>
            </p>
        <p>
                    You stop replication by closing your environment
                    cleanly, as is normal for an DB application.
            </p>
        <p>
                For example, the following code fragment initializes, then
                stops and starts replication. Note that other replication
                activities are omitted for brevity.
            </p>
        <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
          <h3 class="title">Note</h3>
          <p>
            Note that the following code fragment would be part of a larger
            class that must implement
            <tt class="classname">com.sleepycat.db.EventHandler</tt>. This
            class is used to track state changes between master and
            replica. We put off that implementation for the moment, but the
            point remains that the following code fragment would be
            contained in a method or two that you would include in your 
            <tt class="classname">com.sleepycat.db.EventHandler</tt> 
            implementation. 
        </p>
        </div>
        <pre class="programlisting">
import com.sleepycat.db.Environment;
import com.sleepycat.db.EnvironmentConfig;
import com.sleepycat.db.ReplicationHostAddress;
import com.sleepycat.db.ReplicationManagerStartPolicy;

...
    String progname = "example_replication";
    String envHome = "TESTDIR";
    int cachesize = 10 * 1024 * 1024;

    Environment dbenv;
    ReplicationHostAddress thisHost;
    String listenHost = "mymachine.sleepycat.com";
    int listenPort = 8080;

    ReplicationHostAddress otherReplica;
    String otherHost = "anothermachine.sleepycat.com";
    int otherPort = 8081;


try {
    // Configure the environment handle
    EnvironmentConfig envConfig = new EnvironmentConfig();
    envConfig.setErrorStream(System.err);
    envConfig.setErrorPrefix(progname);
    envConfig.setCacheSize(cachesize);
    envConfig.setTxnNoSync(true);

    // Identify the local replication site. This is the local hostname and 
    // port that this replication participant will use to receive 
    // incoming replication messages. Note that this can be
    // performed only once for the application. It is required.
    thisHost = new ReplicationHostAddress(listenHost, listenPort);
    envConfig.setReplicationManagerLocalSite(thisHost);

    // Set this application's priority. This is used for elections.
    // 
    // Set this number to a positive integer, or 0 if you do not want
    // this site to be able to become a master.
    envConfig.setReplicationPriority(100);
    // Add a site to the list of replication environments known to 
    // this application.
    otherReplica = new ReplicationHostAddress(otherHost, otherPort);
    envConfig.replicationManagerAddRemoteSite(otherReplica);

    // Identify the number of sites in the replication group. This is
    // necessary so that elections and permanent message
    // handling can be performed correctly.
    envConfig.setReplicationNumSites(2);

    // Configure the environment's subsystems. Note that we initialize
    // replication. This is required.
    envConfig.setAllowCreate(true);
    envConfig.setRunRecovery(true);
    envConfig.setThreaded(true);
    envConfig.setInitializeReplication(true);
    envConfig.setInitializeLocking(true);
    envConfig.setInitializeLogging(true);
    envConfig.setInitializeCache(true);
    envConfig.setTransactional(true);

    // Missing from this is where we set the event handle and the
    // acknowledgement policy. We discuss these things later in this 
    // book. 

    // Open our environment handle.
    try {
        dbenv = new Environment(envHome, envConfig);
    } catch(FileNotFoundException e) {
        System.err.println("FileNotFound exception: " + e.toString());
        System.err.println(
            "Ensure that the environment directory is pre-created.");
    }

    // Start the replication framework such that it has three threads.
    dbenv.replicationManagerStart(3,
        ReplicationManagerStartPolicy.REP_ELECTION);

    //////////////////////////////////////////////////
    // All other application code goes here, including
    // database opens.
    //////////////////////////////////////////////////

} catch (DatabaseException dbe) {
    // Error handling goes here
}

// Close out your application here.
try {
    // Make sure all your databases are closed.

    // Closing your environment stops replication.
   dbenv.close();
} catch (DatabaseException dbe) {
    // Error handling here.
}

// All done.  </pre>
        <div class="sect2" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h3 class="title"><a id="election_flags"></a>Managing Election Policies</h3>
              </div>
            </div>
            <div></div>
          </div>
          <p>
                Before continuing, it is worth taking a look at the

                

                <span>
                        startup election options that you can set for
                        replication. You set these using the
                        <tt class="classname">ReplicationManagerStartPolicy</tt> 
                        class that you pass to the
                        <tt class="methodname">Environment.replicationManagerStart()</tt>
                        method.
                        
                </span>
            </p>
          <p>
                    In the previous example, we specified 
                        
                        <tt class="literal">ReplicationManagerStartPolicy.REP_ELECTION</tt>
                    when we started replication. This causes the
                    application to try to find a master upon startup. If it
                    cannot, it calls for an election. In the event an
                    election is held, the environment receiving the most number of
                    votes will become the master.
            </p>
          <p>
                    There's some important points to make here:
            </p>
          <div class="itemizedlist">
            <ul type="disc">
              <li>
                <p>
                                    This 
                                     
                                    <span>option</span> 
                                    only requires that other
                                    environments in the replication group
                                    participate in the vote. There is no
                                    requirement that
                                    <span class="emphasis"><em>all</em></span> such
                                    environments participate. In other
                                    words, if an environment
                                    starts up, it can call for an
                                    election, and select a master, even 
                                    if all other environment have not yet
                                    joined the  replication group.
                            </p>
              </li>
              <li>
                <p>
                                    It only requires a simple majority of
                                    participating environments to elect a master. The number of
                                    environments used to calculate the simple
                                    majority is based on the value set for

                                    
                                    
                                    <span><tt class="methodname">EnvironmentConfig.setReplicationNumSites()</tt>.</span>
                                    
                                    
                                    This is always true of elections held using the replication framework.
                            </p>
              </li>
              <li>
                <p>
                                    As always, the environment participating in the election with the most
                                    up-to-date log files is selected as
                                    master. If an environment with better log files
                                    has not yet joined the replication
                                    group, it may not become the master.
                            </p>
              </li>
            </ul>
          </div>
          <p>
                    Any one of these points may be enough to cause a
                    less-than-optimum environment to be selected as master.
                    Therefore, to give you a better degree of control over
                    which environment becomes a master at application startup,
                    the replication framework offers the following start-up
                                     
                                    <span>options:</span> 
            </p>
          <div class="informaltable">
            <table border="1" width="80%">
              <colgroup>
                <col />
                <col />
              </colgroup>
              <thead>
                <tr>
                  <th>Option</th>
                  <th>Description</th>
                </tr>
              </thead>
              <tbody>
                <tr>
                  <td>
                    <tt class="literal">ReplicationManagerStartPolicy. REP_MASTER</tt>
                  </td>
                  <td>
                    <p>
                   The application starts up and declares itself to be a master
                   without calling for an election. It is an error for more
                   than one environment to start up using this flag, or for
                   an environment
                   to use this flag when a master already exists. 
                   </p>
                    <p>
                           Note that no replication group should
                           <span class="emphasis"><em>ever</em></span> operate with more than
                           one master.
                   </p>
                    <p>
                           In the event that a environment attempts to become a
                           master when a master already exists, the
                           replication code will resolve the problem by
                           holding an election. Note, however, that there
                           is always a possibility of data loss in the face
                           of duplicate masters, because once a master is
                           selected, the environment that loses the election will
                           have to roll back any transactions committed
                           until it is in sync with the "real" master.
                   </p>
                  </td>
                </tr>
                <tr>
                  <td>
                    <tt class="literal">ReplicationManagerStartPolicy. REP_CLIENT</tt>
                  </td>
                  <td>
                    <p>
                                  The application starts up and declares
                                  itself to be a replica without calling for
                                  an election. Note that the application
                                  can still become a master if a subsequent
                                  application starts up, calls for an
                                  election, and this application is elected
                                  master.
                          </p>
                  </td>
                </tr>
                <tr>
                  <td>
                    <tt class="literal">ReplicationManagerStartPolicy. REP_ELECTION</tt>
                  </td>
                  <td>
                    <p>
                          As described above, the application starts up,
                          looks for a master, and if one is not found calls
                          for an election.
                          </p>
                  </td>
                </tr>
                <tr>
                  <td>
                    <tt class="literal">ReplicationManagerStartPolicy. REP_FULL_ELECTION</tt>
                  </td>
                  <td>
                    <p>
                                  Identical to
                                  
                                  <tt class="literal">ReplicationManagerStartPolicy. REP_ELECTION</tt>
                                  except that the election requires all
                                  known members of the replication group to
                                  participate. If a given environment has not yet
                                  started but it is included in the
                                  replication group count (using
                                  
                                  
                                  <span><tt class="methodname">EnvironmentConfig.setReplicationNumSites()</tt>)</span>
                                  then a master can not be elected.
                          </p>
                  </td>
                </tr>
              </tbody>
            </table>
          </div>
        </div>
        <div class="sect2" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h3 class="title"><a id="thread_count"></a>Selecting the Number of Threads</h3>
              </div>
            </div>
            <div></div>
          </div>
          <p>
                    Under the hood, the replication framework is threaded and you can
                    control the number of threads used to process messages received from
                    other replicas. The threads that the replication framework uses are:
            </p>
          <div class="itemizedlist">
            <ul type="disc">
              <li>
                <p>
                                    Incoming message thread. This thread
                                    receives messages from the site's
                                    socket and passes those messages to
                                    message processing threads (see below)
                                    for handling.
                            </p>
              </li>
              <li>
                <p>
                                    Outgoing message thread. Outgoing
                                    are performed in whatever thread
                                    performed a write to the database(s).
                                    That is, the thread that called, for
                                    example, 
                                    
                                    
                                    <tt class="methodname">Database.put()</tt>
                                    is the thread that writes replication messages
                                    about that fact to the socket. 
                            </p>
                <p>
                                    Note that if this write activity would
                                    cause the thread to be blocked due to
                                    some condition on the socket, the replication framework
                                    will hand the outgoing message to the
                                    incoming message thread, and it will
                                    then write the message to the socket.
                                    This prevents your database write
                                    threads from blocking due to abnormal
                                    network I/O conditions.
                            </p>
              </li>
              <li>
                <p>
                                    Message processing threads are
                                    responsible for parsing and then
                                    responding to incoming replication
                                    messages. Typically, a response will
                                    include write activity to your
                                    database(s), so these threads can be
                                    busy performing disk I/O.
                            </p>
              </li>
            </ul>
          </div>
          <p>
                    Of these threads, the only ones that you have any
                    configuration control over are the message processing
                    threads. In this case, you can determine how many
                    of these threads you want to run.
            </p>
          <p>
                    It is always a bit of an art to decide on a thread count,
                    but the short answer is you probably do not need more
                    than three threads here, and it is likely that one will
                    suffice. That said, the best thing to do is set your
                    thread count to a fairly low number and then increase
                    it if it appears that your application will benefit
                    from the additional threads.
            </p>
        </div>
      </div>
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